Estrogen replacement therapy has been demonstrated to be an effective means of maintaining bone mass in the postmenopausal woman at risk for osteoporosis. Estrogen appears to inhibit bone resorption yet the precise target tissue(s) and mechanism of action for this effect remains unresolved. The specific aim is to determine if estrogen is directly affecting the bone forming (osteoblast) and/or bone resorbing (osteoclast) cells via a high affinity receptor or possibly some type of nonreceptor mediated phenomenon. Alternatively, it is proposed that estrogen's action on bone is mediated through osteoblast and osteoclast precursors and thymus derived lymphocytes. A paucity of data exist suggesting that bone cells do not contain estrogen receptors, but new insight into the cellular location of the receptor prompts a reassessment, especially based upon the profound effects of estrogen in preventing osteoporosis. Thus, experiments have been designed to assess these various cell types (osteoblasts, osteoclasts, monocytes, macrophages and T-lymphocytes) for the presence of a high affinity estrogen binding site. The direct response of osteoblasts and osteoclasts to estrogen will be monitored by evaluating the de novo synthesis of proteins, mRNA in vitro translation products, specific cDNA probe (Type I collagen and plasminogen activator) hybridization with isolated RNA, and progesterone receptor induction. The indirect effect(s) of estrogen will be tested by utilizing conditioned media (potentially containing various growth and differentiating factors) from monocyte and lymphocyte cultures treated with estrogen. Conditioned media will be added to monocyte, osteoblast and osteoclast cultures and their response appropriately assayed by induction to differentiate, Type I collagen mRNA transcription and 45Ca resorption from mouse calvaria, respectively. These experiments are to elucidate the role estrogen plays in the bone remodeling process. What are the estrogen target sites? How do they respond? How is the response related to bone formation and resorption? This information should settle the issue of how estrogen preserves bone and may lead to improved prevention of osteoporosis, new methods of treatment reducing potential risks of hormonal therapy and clarify several unanswered questions involving the intricate relationship of bone cells, their precursors and thymus derived lymphocytes.